“The brethren direct me to say that the Church has never announced a definite doctrine upon this point. Some of the brethren have held the view that it was possible in the course of progression to advance from one glory to another, invoking the principle of eternal progression; others of the brethren have taken the opposite view. But as stated, the Church has never announced a definite doctrine on this point.”
Secretary to the First Presidency in a 1952 letter; and again in 1965
(No online official source)